Lenovo ideapad s145 upgrade hard drive?

If it's new then take it back and buy or exchange it for one with a larger hard drive already installed.It will cost you more if you do this after. You will have to transfer everything from one hard drive to another and you will pay about $150 in labor plus the new hard drive. Then you will have an extra hard drive just laying around and you will get less for it if you try to sell it Go back to where you bought it and ask them for recommendation on what size depending on what you plan to use the laptop for.8gb should be plenty

1) You could return for a model you want.
2) You can buy a 2.5" SATA HDD or SSD of higher capacity than all data on the laptop drive and SATA-USB enclosure. Install free HDD Cloning software. Clone the data from laptop storage to the external drive. Swap the two drives. You will have the new drive as a copy of the old one in the laptop and the old one from the laptop in the enclosure.
3) The S145 appears to have an unused M.2 drive but missing a hold-down screw. You can add a SATA M.2 SSD into the open M.2 slot. Confirm the M.2 is empty first by opening the back of the laptop and looking at it. I do not know if the screw comes with a purchased M.2 drive nor specifications of the little screw. This will take some additional research. After putting in the 2nd drive (SSD), and rebooting, Windows should recognize it in WIndows explorer as an extra storage probably letter D. To make the SSD the primary drive, you would clone C to D and make D the boot drive, possible also changing Path commands. This is all easier from a hardware view and adds a 2nd internal drive, but you need to get details about the little screw and buy one, and also help to setup and use the SSD for its best uses.

Check memory suppliers(who also stock drives)

You could always get an external USB HD or SSD. Much more convenient than changing the internal hard drive.